1. Field of the Invention
The present invention relates to a refractometer that is equipped with an optical pattern-beam projection system which projects a pattern light beam onto the fundus of a subject's eye to measure the refractive power of the eye and an optical light-receiving system which receives the pattern light beam reflected at the fundus and computes the refractive power of the eye.
2. Description of the Related Art
In a conventional refractometer, a ring-shaped pattern light beam for measuring the refractive power of a tested eye is projected by an optical pattern-beam projection system. The ring-shaped pattern light beam is reflected at the fundus and received as a pattern image with a light receiving element. In this way, the refractive power of the tested eye is measured.
In a refractometer such as this, when a tested eye has normal vision (.+-.0 diopter), a pattern image 1' with a ring shape and a predetermined size is projected on a light receiving element, as shown in FIG. 9. When a tested eye has farsightedness, the pattern image is larger than that in normal vision. Also, when a tested eye has nearsightedness, the pattern image is smaller than that in normal vision.
The center positions P1 and P2 of the width of the pattern image 1' projected on the light receiving element are detected by the peak positions of a light quantity distribution. Based on the detected peak positions P1 and P2, the inter center distance L of the pattern image 1' (i.e., distance between P1 and P2) is computed to obtain the refractive power of a tested eye.
However, in the conventional refractometer, for example, when a vein or scar 2 exists at the measured position of the inter center distance on the pattern light beam 1 projected on the fundus of a tested eye, as shown in FIG. 10, a portion corresponding to the scar 2 is obtained as a peak position, and a center position P1' shifted from an actual center position P1 is detected. For this reason, there arises the problem that normal vision is measured as astigmatism by mistake.